Summary
Activated protein C (aPC) proteolytically inactivates factor Va (FVa) and thereby
downregulates prothrombinase. Although FVa inactivation by aPC has been studied extensively,
the inactivation of prothrombinase during prothrombin activation has not. Therefore,
prothrombin activation initiated both without and with aPC (5.0, 7.5 or 10.0 nM) was
monitored over time by fluorescence. The experiments were performed with 0.075 nM
FVa and 1.0 nM FXa, and with these concentrations reversed. The time courses of the
residual prothrombinase activity with aPC, determined from the slopes of fluorescence
over time, were pseudo first order with both limiting and excess FVa. With FVa limiting
or in excess, the second rate constants for inactivation of prothrombinase were 1.98
± 0.09 x 105 M-1s-1 and 2.54 ± 0.13 x 105 M-1s-1 , respectively. The former value is 101-fold smaller than that for FVa inactivation
by aPC alone. Since with limiting FVa the second order rate constants for prothrombinsase
inactivation and FVa inactivation are equal, FVa is protected 101-fold, presumably
by both FXa and prothrombin. In contrast, with excess FVa, the calculated rate constant
for FVa inactivation exceeds that for prothrombinase inactivation 17.3-fold, which
reflects a loss of protection by FXa. Since the protective effects of the two proteins
are theoretically multiplicative, FXa protected 17.3-fold and prothrombin protected
5.8-fold. With 150 nM protein S and limiting FVa, prothrombinase inactivation was
two-fold faster, yet it was still protected 91-fold. These studies show that FVa is
down-regulated by aPC during prothrombin activation, but both FXa and prothrombin
protect FVa in a multiplicative way, with or without protein S.
Keywords
Coagulation factors - protein C/S pathway - regulation